Automatic volume control time response switching



E. H. METER 2,582,714

AUTOMATIC VOLUME CONTROL TIME RESPONSE swITcHING Jan. l5, 1952 l T E m ss T E m S 2 A.' 9 umFmOZS/JQO 0\ 1 @L 5. l u A d e l .l F

rzv enter' dwefr H.

MJQSNS E. H. MEIER Jan. 15, E952 AUTOMATIC VOLUME CONTROL TIME RESPONSESWITCHING Filer.` Aug. 13, 1949 2 SHEETS- SHEET 2 Nm um MUNO@ UZTQTP. Z.I Z. 5.0M"-

Edw n H. meier Gru/enter" w@ n r w b G Patented Jan. l5, 1952 AUTOMATIC"OLUME CONTROL TIME RESPONSE SWITCHING Edwin H. Meier, Tulsa, Okla.,assigner to Standard Oil Development Company, a corporation of DelawareApplication August 13, 1949, Serial No. 110,189

The present invention relates to an improved apparatus and method forobtaining seismograph records in seismic prospecting. More particularlythe invention relates to the obtaining of seismograph records in whichthe recorded traces are of the same order of magnitude throughout therecord.

One well-known technique of exploration for 011 or other mineraldeposits comprises seismic prospecting or reilection seismographywherein a hole is drilled into the earth and an explosive shot or othermeans of producing sound is placed in the hole. The sound source isactuated and the sound waves traveling through the earth are detected atvarious points on the .surface of the earth by means of sensitivepick-ups or geophones which translate the detected sound waves intoelectrical impulses which after suitable amplification can be recordedon a seismograph.`

7 claims. (ci. 179-171) i evident that in order that the traces on theseismograph record may be of usable size, the amplier circuit associatedwith each geophone andits corresponding galvanometer must have an ingthe output reasonably constant over the above mentioned range of inputsignals.

One suitable method of producing the desired range of amplitude controlin a seismograph amplifier circuit is to provide for one or more stagesof xed gain amplication and filtering together ters each one of whichhas a mirror attached thereto, the galvanometers being arranged in abattery in connection with a source of light in such relation to amoving strip of sensitized paper or lm that there will be recorded onthe paper or lm a plurality of wave forms or traces representative ofthe sound waves which have been picked up by the individual geophones,suitably amplified, and fed to the galvanometers. Many seismographinstruments are capable of recording as many as 24 or 36 individualtraces simultaneously. The strip of paper or lm is moved longitudinallyat a substantially constant speed and is provided with suitable timingmarks so that when the seismograph record or seismogram is laterexamined it is possible to determine the length of time required for thearrival of sound waves at any particular point on the earths surfaceeither directly from the sound source or by reflection from underlyingstrata.

During the making of a seismograph record the rst kick, i. e., the rstarrival of the disturbance from the shot, will generally reach thegeophones placed near the shot point within a few hundredths of a secondafter the shot has been detonated and the ground disturbance willcontinue for from about one to four seconds until it decreases to thelevel of normal ground unrest. The amplitude of the first kick signal,which represents the upper level of wave energy force that it is desiredto detect may diier from the amplitude of normal ground unrest,representing the lower level of wave energy to be detected, by a ratioof as much as a million to one. Itis thus with an amplification stagehaving variablevgain in conjunction with an automatic volume controlloop. The present invention is concerned with the provision of betterautomatic volume control in connection with such amplification.

Since the first impulse (commonly referred to as the rst kick) receivedby the geophones after shooting during seismic prospecting is of fairlylarge amplitude it is desirable to provide for reducing the gain of theamplier rapidly at this time forV increasing signals in order to get therecorded trace under control within a few.

gain will not be reduced toovrapidly when reflected impulses arereceived, thus permitting the reflected impulses to persist withoutbeing distorted by changes in gain during reception of the reflectedimpulses.

It is an object of the present invention to provide for improvedautomatic amplitude control, in an amplifier circuit used in seismicrecording.. It is another object of the invention to provide..

for switching of the time response characteristics of the amplifiercircuit so that the amplitude of the seismograph trace can be broughtunder. control rapidly and then maintained at a relatively constantlevel for the rest of the trace without reducing the amplitude ofreflection signals that will be superimposed on the general signallevel. A more specific object of the invention is to provide anautomatic volume control circuit,

in a variable gain amplifier for a seismograph which will contain meansfor time response switching during seismic recording, furnishing fastresponse time for gain control initially followed, after receipt of thefirst impulse, by modto actuate the time response switching inztheicir-Ycuit of Fig. 1.

With particular reference lto Fig. 1, afseismic signal such as onereceived 'from a geophone is fed into a fixed amplier and filter networkI and thence into a variable gain amplier 2. The output from amplifier 2is fed to transformer 3 and thence to the recording galvanometer of theseismograph.

Fixed amplifier and filter network I may be of any'suitable design,its'function being toarnplify the signal vto a level suitable forrecording on a Aseismograph land for ltering out spurious signals whichdo not contribute to interpreta-1 tion of the record. A suitable amplierlter network will be one providing for ampliiicationfactors of say 100to 3000 and which is adapted to filter out signals whose frequencies areAoutside of -those desired for the record. Thus ground roll, havingfrequencies below about l cycles per second, and wind noise, havingfrequencies of 100 cycles per second or higher, would be excluded.

Variable gain amplifier 2 can be of any design providing for automaticcontrol of gain'as a function of signal level. For example, it maycomprise an amplier stage in conjunction with an attenuator stagewherein the latter is adapted to reduce the -amplitude of signals fedinto it in proportion to a control voltage applied thereto. One suitabledesign of attenuator comprises a voltage divider consisting of a fixedresistanceV and an electronic circuit comprising two vacuum tubes whoseplates are connected to each other, a Ygrid of one of the vacuum tubesbeing tied to the cathode of the second tube and to the fixed resistanceand the cathode of the rst vacuum tube being connected to a grid of thesecond vacuum tubevand to the common ground of the circuit.Attenuation-is controlled by the amount of negative potential applied toan electrode in the circuit other than a cathode and other than theelectrode connected Yto a cathode. This type attenuator circuit isdescribed and `claimed in copending application Ser. No. 98,506 of EdwinH. Meier, filed June 11, 1949.

'Referring further to Fig. 1, by means of a third winding 4 ontransformer 3 Ypart of the output signal is sampled and fed to inputterminals 6 and 'I of full wave bridge rectifier 5. The output ofrectifier 5 is adapted to be -fed by means of relay-operated switches I9and 20 through either `of two filter-amplifier arrangements. a position,which is the position of the switches until .after the first kick isreceived from the seismic pickup, the lter arrangement is such that thegain of amplifier 2 is reduced rapidly when the signal suddenly appears,but for de creasing signal levels gain control is subjected to a longtime delay. i

After .the rst kick has been received switches I9 and 20 are thrown tothe b position by means of relays as described more fully later in Wheneach of these switches is in the' connection with Fig. 2. With theswitches in the b position the time response for gain control ismoderately fast for decreasing signal level and moderately slow for gaincontrol of increasing signal level.

Thus it may be stated that the invention comprises an .improvedautomatic volume control circuit for the variable gain amplifier vstageof an amplifier circuit in which the automatic vol- :ume control circuitcomprises a first time response vnetwork vcharacterized by having afirst time `.delay for bringing the gain of the variable gain amplifierstage under control when a signal of .increasing amplitude is receivedby the amplifier "stage andaa'secondltime delay for controlling the gainforV signals of )decreasing amplitude and a -seconditime responsenetwork characterized by having a third time delay for bringing the gainunder control when a signal of increasing amplitude is received and afourth time delay for gainxcontrol Vfor signals of decreasing amplitude,together withimea-nsifor switching from thefirs't time response networktto the .second time response network afterz anvinitial impulse isreceived by the amplifier circuit. The .first time delay is short,1thesecond one is long, and the third one moderately long and the fourth onemoderately short. In other words, the fourth time delayis :longer'athanthe.:irst,'ethe third time delay'is longer than the fourth and thesecond is .longer than the third. Furthermore the longest time delay, i.e., the second, is of the order of :magnitude of 50 to. 100 'times thatof the rst.

' In general, in the firsttime `.response network, i.- e., that whichisoperative when :switches i9 and '20 are' in fthe "a or ".rst kickposition, the rst gain .control vtime delay (for increasing signallevel) should be in the range of about 0.01

to 0;03 second (preferably about 0.02 second) and the second time delay(for decreasing signal level) should bein the range of about 0.5 to 2seconds (preferably about 1 second). In the second time lresponsenetwork, operable when switches .I9 and '20 are inthe b or reflectionposition, the third Atime delay (for increasing signallevel) shouldgenerally lie in the range of about 0.3 .to .0.7 second (preferablyabout 0.5 second). andthe fourth time delay (for decreasing signallevel) in the range of about 0.05 to 0.2 second (preferably about 0.1second).

Time 'response .as Ythe term is used here is defined as the timeVrequired forthe output voltage to return essentially to its previouslevel after the input voltage `has been increased or decreasedinstantaneously by a factor of 10.

In .the example of a suitable arrangement for effecting gain controltimedelay as depicted in Fig. 1 negative output terminal 9 of rectifier5 is connected through resistor I2 to the grid of D. C. amplifier tubeIII and to condenser I6. Output terminal .9 is lalso connected to thearm of switch 20. The plate .of amplifier tube I9 is connected tocondenser I6 and through resistor I3 to the control `grid of' cathodefollower tube II. There is alsova connection from 'the plate of tube I0through resistor I4 to the positive bus 23 of the source of B voltagefor the circuit. Positive terminal 8 of rectifier 5 is .connected to thenegative bus 2-I of the'source of B voltage as are the cathodes ofytubes III and I I, the latter connection being through resistor I5. Thecathode and suppressor grid of tube II are connected through condenserI1 to switch point a of Switch 20. Switch point b of switch 20 makes ankopen circuit. The screen grid and 'plate of tube II arei tied to thecommon ground of the circuit. Condenser I8 is tied from the controlpoint in variable gain amplifier stage 2 to the com-mon ground. Tube IIis hooked up as a triode and obviously could be replaced by a triodetube. When switch.

that they will be operated simultaneously or they may constituteelements of a single switch with the proper contact points.

It is to be understood that the various electrical components of theautomatic volume control circuit depicted in Fig. 1 may have valuesselectedA over :a fairly wide range. As a specific example of anoperable combination, tube I may be a 6AQ6 triode, tube II a 9001pentode, resistors I4 and I lmay have values of 5 megohms, resistor I2of 0.5 megohm and resistor I3 of 0.3 megohm. Condensers I6, I'I and I8would have capacities of 0.001, 0.1 and 1 microfarad, respectively. Bvoltage busses 2I and 23 may suitably be -180 volts and +180 volts whenthe above components are used.

Resistor I2 and condenser I6 serve as a filter' for the rectifiedvoltage received from rectifier 5 and impressed on the grid of ampliiiertube I0. In general, the characteristics of this filter should be suchthat it will pass essentially all frequencies below about 15 cycles persecond and will cut out practically all ripples above' about 25 to 30'cycles per second. Such characteristicscan be obtained by selectingresistorv and condenser values such that the mathematical product of theamplification factor of tube I0, the resistance in ohms of resistor I2Vand the capacitance in farads of condenser I6 lies in the range of about0.01 to 0.05. Thus if tube IIJ has an amplification factor of 50,resistor I2 a resistance of 0.5 megohm and condenser I6 a capacitance of0.001 microfarad, the product will be 0.025.

In order that the automatic volume control circuit will have the propertime response characteristics it is desirable that the various otherresistances and capacitances in the circuit havel the followingmathematical relationships, resistances being expressed in ohms andcapacitances in farads:

(1) For the rst kick position of the circuit, i. e., when switches I9and 20 are in the a position, the product of the capacitance ofcondenser I8 (the condenser tying the control point of amplifier stage 2to ground) and the plate resistance of cathode follower tube I I shouldlie in the range of about 0.01 to 0.05, and the product 0f thecapacitance of condenser I8 and the resistance of cathode resistor I5should be at least 1.

(2) For the reflection position of the circuit,-

i. e., when switches I8 and 20 are in the b position, the product of thecapacitanceof condenser I 8 and the sum of the resistance of resistor I3and I Il should be at least 1 and the product of the capacitance ofcondenser I8 and the sum of the resistance of resistor I3 and the plateresistance of the amplifier tube I0 should be in thev range of about 0.1to 0.5. The actual values will of course depend on the requirement ofthe lparticular circuit used.

The function of condenser I`I is to stabilize the circuit to keep thegain control from overshooting when the rst impulse is received. ItwillV be seen that this condenser is cut out of the cir-y cuit when theswitches I9 and 20 are thrown to the b position. Condenser I1 may have acapacitance value of the range of about 0.02 to 0.5 microfarad,depending on the needs of the specific circuit.

Referring again to Fig. l it `will be seen that with switches I9 and 20in the a position the rectified voltage obtained from rectifier 5 isfiltered to some extent and is used to furnish a voltage on the grid oftube I0 which is negative with respect to the potential on the cathodeof that tube. The voltage on the plate of tube I0 is impressed on thecontrol grid of tube II and thus determines the voltage applied to thecontrol point of variable gain amplifier stage 2. When a sudden increasein signal level occurs in the amplifier output the voltage impressed onthe grid of tube I0 suddenly increases and tube I0 is cut ofi, causing apositive signal to appear suddenly on the control grid of tube II. Thelow impedance output of the cathode of tube II rapidly changes thecharge on condenser I8 and thus gives a fast time response to thecontrol voltage applied to amplier stage 2. When the output signal levelis decreasing the time response is determined by cathode resistor I 5and condenser I8 and is relatively long.

After switches I9 and 20 have been thrown to position b after arrival ofthe rst impulse from the seismic pickup, as described in conjunctionwith Fig. 2, it will be seen that the plate of tube I0 is connected tothe control point of amplifier stage 2 through resistor I3. When anincreased output signal level occurs which is reflected as an increasedpotential on the grid of tube I0, tube Ill is cut oli and condenser I8is charged moderately slowly toward the plus potetial of busr23 throughresistors I3 and I4, giving an intermediate time response to the controlvoltage applied to amplier stage 2. When a decrease in output signallevel occurs, current flows through tube I0 and condenser I8 is chargedtoward the negative potential of bus 2I through resistor I3 at amoderately fast rate, giving a relatively short time response to thecontrol voltage applied to stage 2.

The manner in which switches I9 and 20 are thrown from position a toposition b is shown in Fig. 2. The initial impulse received from aseismic pickup placed near the origin of the seismic shot is fed intoamplifier 3 I. The amplified impulse is fed into the grid of a thyratron32 which in turn operates an initiating relay 33. 'Ihe latter relay maybe adapted to change the potential on delay circuit 34 comprising aVresistance-capacitance network to delay the buildup of a potential onthyratron 35. When a suiiicient charge has been built up to re thyratron35, relay 36 is actuated to throw switches I9 and 20 simultaneously. Bya suitable choice of resistance-capacitance values in circuit 34 theactuation of switches I9 and 20 can be eiected at any desired time afterreceipt of the initial impulse in initiating amplifier 3|.

Alternatively switches I9 and 20 could be hand controlled or could beactuated by a delay relay by well known means.

The invention is not limited to the specific examples presented, asnumerous modifications thereof can be made by those skilled in thisvparticular art. The invention is to be limited only by the followingclaims.

What is claimed is:

1. In an amplifier circuit forv seismic signals received from alseismi'cpickupfand fed to a seismograp'h, `said circuit including a variablezgain amplifier stage wherein Ygain is controlled by a voltage impressedon 'a control point therein, an improved Aautomatic volume controlcircuit for said stage comprising means for sampling the signal outputvoltage from Jsaid variable gain amplifier Stagemeans for rectifying thesampled output voltage, means for amplifying lthe rectified voltage, a'first timelresponse means adapted to feed-said Arectified voltage ontosaid control point, adapted to impose a Afirst short time delay intransmitting to said control point voltage changes `representative ofincreased signal level and adapted to vimpose a second long time delayin transmitting to said control point voltage changes representative ofdecreased signal level, a second time response means adapted to feedsaid rectified voltage onto said control means, adapted to 4impose athird moderately long time delayfin .transmitting to said control pointvoltage changes representative of increased signal level and adapted toimpose a fourth moderately short time delay in transmitting to saidcontrol point voltage changes representative of decreased signal level,saidfourthmoderately short time delay being longer than said 'firstshort time delay, said third moderately long time delay being longerthan said fourth moderately short time delay and said second long timeYdelay being longer than 'said third moderately long time delay,switching means having a first position in which connection visestablished between said first time response means, said rectifiedvoltage amplifying means `and said control point and having a secondposition in Vwhich vconnection is established between said second timeresponse means, said last named amplifying means and 'said control pointAand means `adapted to move said switching means from its 4firstposition to its second position after a predetermined interval followingreceipt of van initial impulse from said seismic pickup.

2. Improved circuit according to claim 1 in which said second time delayhas a duration of from 50 to 100 times said first time delay.

3. Automatic volume control circuit according to claim l in which insaid first time response means said rst short time delay is from about0.01 to about 0.03 second and said second long time delay is from about0.5 to about 2 seconds and in which in said second time response meanssaid third moderately long time delay is from about 0.3 to about 0.7second and said fourth moderately short time delay is from about 0.05 toabout 0.2' second.

4. In an amplifier circuit for seismic signals received from a seismicpickup and fed to a seismograph through an output transformer, saidamplifier circuit including a variable gain amplifier stagev wherein:gain is controlled by a voltage impressed on a control point therein,an improved automatic volume control circuit comprising a samplingwinding on said output transformer for sampling the output signalvoltage from said variable gain amplifier stage, a full wave bridgerectifier, means connecting the input of said rectifier with saidsampling winding, a D. C. amplier tube, a cathode follower tube, meansconnecting the output of said rectifier through a filter to the grid ofsaid D. C. amplifier tube, means -connecting the plate of said D. C.yamplifier tube with the control grid of said cathode follower tube,switching means having a first contact position and a -second contactposition, said switching means being adapted when in said first contactposition to connect the vvcathode of said cathode follower tube to thecontrol Vpoint of said variable gain amplifier stage Aand when in saidsecond position to connect the plate of said D. C. amplifier tubevthrough a resistor to the control point of said variable gain amplifierstage, means adapted to move said switching means from its first contactposition to its second contact position after a predetermined intervalfollowing receipt of a first impulse from said seismic pickup, and meansconnecting said control point of said variable gain amplifier stage tothe 'common ground of said circuit through a condenser.

5. Circuit according to claim 4 in which said filter is adapted to passessentially all frequencies below about 15 cycles per second and tocutout essentially all ripples above about 25 to 30 cycles per second,and in which the capacitance in farads of Said condenser connecting saidvariable gain control point with the common ground of the circuit bearssuch mathematical relationship with the various resistances in thecircuit in ohms that the product of said capacitance and theresistancefof the plate of the cathode follower tube lies in the-rangeof about 0.01 to 0.05, the product of said capacitance and theresistance of the cathode resistor of the cathode follower tube is atleast 1, the product of said capacitance and the sum of the resistanceof the plate resistor of the D. C. amplifier tube and the resistance ofthe resistor connecting the plate of the D. C. amplifier tube with theYvariable gain stage control point is at least 1, and the product of saidcapacitance and the sum of the resistance of said last named resistorand the resistance of the plate 0f said D. C. amplifier tube lies in therange of about 0.1 to 0.5.

6. In anfamplifier circuit for seismic signals received from a seismicpickup and fed to a seismograph through an output transformer, saidamplifier circuit including a variable gain amplifier stage wherein gainis controlled by a voltage impressed on a control point therein, animproved automatic volume control circuit comprising a sampling windingon said output transformer for sampling the output signal voltage fromsaid variable gain amplifier stage, a full wave bridge rectifier, meansconnecting said sampling winding with the input of said rectifier, afirst vacuum tube, a 'first resistor connecting the control grid of saidfirst vacuum tube with the negative output terminal of said rectifier, afirst condenser connecting the control grid of said first vacuum tubewith the plate of said first vacuum tube, a second vacuum tube, a secondresistor connecting the plate of said first vacuum tube with the controlgrid of said second vacuum tube, a source of B potential, meansconnecting the cathode of said first vacuum tube with the negative sideof said source of B potential, a third resistor connecting the plate ofsaid rst vacuum tube with the positive side of said source of Bpotential, a fourth resistor connecting the cathode of said secondvacuumtube with the negative side of said source of B potential, meansconnecting the positive side of said rectier with the negative side ofsaid source of B potential, a vsecond condenser connecting the controlpoint of said fixed gain amplifier stage with the common ground of thecircuit, means connecting the plate of said second vacuum tube with thecommon ground of the circuit, a third condenser connected to the cathodeof the second vacuum tube, a first switching c means having a firstposition and a vsecond position and adapted in itsfirst position toconnect the cathode of said second vacuum tube with the negativeterminal of said rectifier through said third condenser and in itssecond position to open said connection, a second switching means havinga first position and a second position and adapted to its rst positionto connect the cathode of said second tube with the control point ofsaid variable gain amplifier stage and in its second position to connectthe plate of said first vacuum tube through said second resistor Withthe control point of said variable gain amplifier stage, and meansadapted to throw said first and second switching means from their firstposition to their second position `after a predetermined intervalfollowing receipt of a rst impulse from said seismic pickup.

'7. Improved circuit according to claim 6 in which the resistance valuesin ohms of the resistors and the capacitance values in farads of i thecondensers employed therein are determined by mathematical relationshipswherein the product of the amplication factor of the first vacuum tube,the resistance of the rst resistor and the capacitance of the firstcondenser lies in the range of 0.01 to 0.05, the product of thecapacitance of the second condenser and the resistance of the plate ofthe second vacuum tube lies in the range of 0.01 to 0.05, the product ofthe capacitance of the second condenser and the resistance of the fourthresistor is at least l, the product of the capacitance of the secondcondenser and the sum of the resistances of the second and thirdresistors is at least 1 and the product of the capacitance of the secondcondenser and the sum of the resistances of the second resistor and theplate of the first vacuum tube lies in the range of 0.1 to 0.5.

EDWIN H. MEIER.

REFERENCES CITED UNITED STATES PATENTS Name Date Fay et al. Nov. 22,1949 Number

